From redox gating to quantized charging.

Electron transport characteristics were studied in redox molecule-modified tunneling junctions Au(111)/6-thiohexanoylferrocene (Fc6)/solution gap/Au STM tip in the absence and in the presence of gold nanoclusters employing an electrochemical STM setup. We observed transistor- and diode-like current-voltage responses accounted for by the redox process at the ferrocene moiety. We demonstrate that the reorganization energy of the redox site decreases with decreasing gap size. As a unique new feature, we discovered the formation of uniform (size approximately 2.4 nm) gold nanoparticles, upon multiple oxidation/reduction cycles of the Fc6 adlayer. The immobilized nanoparticles modify the electron transport response of the Fc6 tunneling junctions dramatically. On top of embedded single nanoparticles we observed single-electron Coulomb charging signatures with up to seven narrow and equally spaced energy states upon electrochemical gating. Our results demonstrate the power of the electrochemical approach in molecular electronics and offer a new perspective toward two-state and multistate electronic switching in condensed media at room temperature.